Background: Adenosine stress CMR relies on adequate vasodilator stress to assess myocardial perfusion. A proportion of patients are inadequately vasodilated using 140mcg/kg/min adenosine, and heart rate rise alone is an inadequate marker of coronary hyperaemia. "Splenic Switch Off" (SSO) has been shown to identify adequate adenosine response due to the A1 A2B receptor mediated vasoconstriction of the spleen, as shown in a sub-analysis of the CEMARC trial. Failed SSO response identifies under-stressed patients who may therefore have false-negative stress perfusion CMR scans. However, not all sites perform rest perfusion imaging in their standard stress CMR protocols. There is a clinical need for a streamlined approach to assess vasodilator efficacy during adenosine stress CMR.

Purpose: We tested a simplified methodology to assess for vasodilator efficacy by comparing peak blood and baseline vs peak splenic signal during adenosine stress CMR.

Methods: 15 patients undergoing clinical stress CMR were retrospectively reviewed under ethics approval. Adenosine was infused at 140mcg/kg/min, and free breathing motion-corrected stress MRI images acquired (Siemens MAGNETOM Aera 1.5T). All patients had SSFP imaging for ventricular function, and late gadolinium enhancement imaging, with no rest perfusion imaging (as per institutional protocol). Patients were dichotomised into having either adequate SSO or non-SSO response based on visual analysis by two SCMR Level 3 readers. Quantitative regions of interest (ROI) were drawn in the blood pool, the myocardium, and the spleen at baseline and peak vasodilation during gadobutrol infusion by a blinded observer using (Circle Cardiovascular Imaging, Calgary, Canada; Version 5.6).

Results: Baseline splenic signal increased on average 2-fold in subjects with adequate SSO response, whereas in non-SSO patients the splenic signal increased 6-fold from baseline (p = 0.0011) indicating failed splenic vasoconstriction. Peak splenic signal expressed as a percentage of peak blood signal was 18% in SSO patients indicating low relative signal from splenic vasoconstriction, but was 72% in non-SSO patients (p = 0.047) indicating lack of splenic vasoconstriction [FIGURE: upper panel note dark spleen compared to blood pool, lower panel note bright spleen]

Conclusion: A simplified approach using ROIs in the blood pool and spleen allows rapid identification of adequate SSO during stress CMR without rest perfusion imaging. This allows opportunity to increase the adenosine dose on-the-fly and potentially reduce false negative scans.